Ideally, relationships are built on a foundation of honesty and trust. If you suspect your girlfriend is lying to you, however, you need to pay attention to your feelings. Girlfriends may obscure the truth for many reasons, but this behavior is always unacceptable in a committed relationship. Pay attention to your instincts. If you suspect your girlfriend is hiding something, you need to address the problem.
Observe your girlfriend's body language when she is on the phone. A person with nothing to hide will be at ease and open when she picks up the phone, hears who is on the other end and converses. A girlfriend who is hiding something usually seems tense, keeps her answers short or leaves the room to talk.
Pay attention when she's on the computer. If she seems secretive about her e-mails or suddenly switches to a different webpage or program when you enter the room, there may be a reason.
When your girlfriend tells you where she's been, listen carefully. Someone who is lying often provides needless details about her destination, companions and what she has been doing in order to make her story sound airtight.
Notice her comings and goings. If your girlfriend disappears at odd times of the day or night, or if certain errands take more time than they should, this may be a sign of deception.
Listen to everything she tells you and notice inconsistencies. Without being heavy-handed, try to verify what she tells you if possible.
Be aware of odd behavior. If your girlfriend is behaving strangely, and her friends and family seem unusually cool or distant towards you, they may be hiding something.